Eddy current inspection is commonly used as a non-destructive method to detect flaws in surfaces of manufactured components fabricated from a conductive material, such as bars, tubes, and special parts for automotive, aeronautic or energy industries.
Typical eddy current sensor configurations are impedance bridge, pitch-catch (alternatively called reflection or transmit-receive) and differential, but can also include more complex combinations such as pitch-catch with differential receivers, multi-differential, etc.
Each eddy current system regardless of its configuration is composed of coils in different shapes. In a standard eddy current testing, a circular coil carrying current is placed in proximity to the test specimen (which must be electrically conductive). The alternating current in the coil generates a changing magnetic field, which interacts with the test specimen and generates eddy currents. Variations in the phase and magnitude of these eddy currents can be monitored using a receiver coil, or by measuring changes to the current flowing in the primary driver coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current and a corresponding change in the phase and amplitude of the measured current.
Eddy current field conditions associated with problematic component conditions (including flaws, conductivity variation, thickness, etc) can then lead to non-destructive examination of the component.
It is always interesting to determine or verify the correct functioning of the eddy current inspection systems; including the entire system acquisition chain, the driver and receiver arrangement, especially when there is an eddy current array with different numbers of coils which makes it difficult to find the broken coils in the system. Since non-destructive testing is typically achieved on critical components, the ability to confirm the correct operation of the inspection system does have a direct impact of the safety level of the inspected component by elimination of risks associated with inspection system malfunctions.
Determining the whole system acquisition chain functionality is especially important and interesting between the inspections. This ensures the correct functioning of inspection system. An integrity check method should be capable of producing a system condition diagnostic in between consecutive inspections of a bar, for example in order to validate that a whole batch of bars has been inspected with a functional system and also to raise an alarm as soon as a failure in the testing equipment is detected.
Determining the probe functionality has been under investigation for many years and there are patented inventions in this field.
U.S. Pat. No. 8,395,377 describes a system for determining the correct functioning of an eddy current probe/cable system. According to that patent, the determination of the status is based on a frequency of the eddy current probe oscillator. The method comprises a plurality of steps. In a first step, a frequency of the eddy current probe oscillator is measured. In a second step, the measured frequency is compared to one or more previously measured frequencies, and/or to a predefined frequency, and/or to a predefined range of frequencies. In a third step, the status is determined by means of the result of the frequency comparison. In another embodiment, the amplitude of the demodulated frequency is measured and compared to one or more previously measured amplitudes, and/or to predefined amplitude, and/or to a predefined range of amplitudes. And that third step of determining the status also comprises determining the status by means of the result of the amplitude comparison.
Although the known method allows determining the correct functioning of the eddy current system, there are some limitations, which restrict the application of this method. First of all, this invention checks only the probe functionality and not the whole acquisition chain. Moreover, the invention needs dedicated probe checking electronics. In addition, the patented method may become difficult to apply to a complex probe design, which includes a multiple coil array.
Another patent (U.S. Pat. No. 8,421,471) describes a self-test unit linked to an eddy current system which performs systematic quantitative checking of the signal processing functions of the signal processing unit, the transmitting coil arrangement, and the receiver coil arrangement, and upon request, calibrates the signal processing unit with a calibration standard which is to replace the transmitter coil arrangement and/or the receiver coil arrangement.
The U.S. Pat. No. 8,421,471 patent asserts that this is advantageous because it allows for comprehensive checking of the functions of the front-end, especially of the filters and amplifiers as well as the probe, and thus, high reliability of the measurement results is achieved, and calibration of the device is also easily enabled. This applies especially to calibration with respect to the adjustable preamplifier.
Nonetheless, the complexity of the U.S. Pat. No. 8,421,471 invention limits its applications. The method requires multiple steps including disconnecting the probe and dedicated electronic and mechanics. In addition, based on its explanation, it seems the U.S. Pat. No. 8,421,471 invention is only suitable to test impedance probes.
Thus, the known probe checking techniques partially meet the need to provide eddy current system integrity diagnostics. Still, a new technique is needed to eliminate the limitations and disadvantages of the known methods, especially for multi-element probes. In particular, there is a need for an eddy current system condition monitor that will be fast enough to be operated between consecutive inspection sequences, that will allow monitoring of the whole acquisition chain including the probe, that could be adapted for multiple probe type including eddy current array and that will be inexpensive.
The following section describes advantages of the presented invention, which eliminate the limitations and drawbacks of the known art described in the foregoing patents.